1. Field of the Invention
The invention is directed generally to communication systems and more specifically to a fault current protection circuit for an auxiliary device for telecommunications terminal equipment.
2. Description of the Related Art
Name keys, electronic notebooks and similar devices are used in digital telephone systems. These auxiliary devices are connected to the basic equipment, i.e. to a telecommunication terminal device, via an interface. For example, telephone sets, fax devices and the like are such telecommunications terminal devices.
Such auxiliary devices generally have a separate dc voltage regulator that the telecommunications terminal equipment supplies with an unregulated dc voltage. The required supply voltage and a reference potential are supplied to the auxiliary devices via a pluggable connecting cable. In addition, the connecting cable contains signal lines for the connection of electronic circuits in the auxiliary device and in the basic equipment.
The risk of a non-uniform contacting due to different plug-in behavior exists given these plug-type connections. When plugging the auxiliary device to the basic equipment under voltage, the protective diodes, commonly referred to as clamping diodes, that are provided at the electronic circuits can be destroyed in the telecommunications terminal equipment and in the auxiliary device. Such a situation occurs when the reference potential is contacted later than the signal lines during plugging or when the reference potential is not contacted at all. In this case, a pseudo return line is formed via the signal lines. The fault current that flows leads to the break down of the protective diodes, which are only designed for an extremely light current load. After the destruction of the protective diodes, the electronic circuits such as microprocessors, integrated circuits and the like can also be damaged.